The invention relates to the field of laser sensitive pigments for laser marking of plastics. Laser marking techniques may be used to replace conventional ink-printing techniques in marking plastic objects for identification or safety purposes, including markings such as date or batch codes, bar codes, and serial numbers. Laser marking is a clean and rapid process which produces permanent, rub-fast, scratch proof, and solvent resistant markings. The laser marking process does not generate waste or involve the use and disposal of hazardous solvents. Laser sensitive pigment additives having broad application with a variety of laser types and resin types would facilitate the laser marking process.
In accordance with the current invention, it has been discovered that laser sensitive pigments comprising particle substrates coated with metal oxides have wide ranging utility in the laser marking of plastics. The laser sensitive pigments may be used in conjunction with a wide variety of plastic compositions and in conjunction with different laser types.
As used herein, laser sensitive pigments means pigments that are capable of absorbing sufficient laser energy to generate a mark on a plastic substrate containing the pigment.
The metal oxide coating comprises at least one marking component which is CuO, MoO3, WO3, V2O5, Ag2O, or PbO2. The metal oxide coating may also comprise a host substrate component of either TiO2 or SnO2 or a mixture of TiO2 and SnO2.
Laser marks on plastic are typically the result of carbonization of irradiated polymer material. In most instances, satisfactory laser marking quality cannot be achieved with the polymer material alone because carbonization does not occur effectively or selectively. The polymer material either does not absorb enough laser energy for carbonization to take place, or the polymer material becomes excessively burned at the surface when laser energy is applied. Laser sensitive additives can enhance the quality of laser marking. However, because the mechanisms behind laser marking are different for different laser types, additives specific to the laser type have typically been employed to obtain optimal laser marking results. CO2 lasers have a laser frequency in the mid-infrared (IR) region (xcex=10600 nm), and preferred additives for CO2 lasers have strong vibrational energy absorption in the mid-IR region and are highly heat resistant. The additives act as secondary heaters that absorb laser energy and raise the temperature of the surrounding polymer to very high levels, resulting in carbonization of the polymer. The laser markings are primarily the result of thermal processes. With ultra-violet (UV) excimer lasers (xcex less than 400), laser marks are mainly realized via photochemical processes. Nd:YAG lasers have wavelength frequencies in between the two extremes for CO2 lasers and UV lasers, with frequencies at 1064 nm (near IR) and at 532 nm (visible) for an optically doubled frequency. For Nd:YAG lasers, both vibrational energy absorption and photochemical processes are necessary to achieve high quality marks.
As discovered herein, certain particle substrates coated with certain metal oxides may be employed as laser sensitive pigments for use in laser marking of plastics using different types of lasers. Sensitivity of the pigments to a particular laser type is primarily attributable to individual components of the coated substrates, although the other components may enhance laser sensitivity and marking contrast. For example, sensitivity to a CO2 laser is primarily attributable to the particle substrate. Sensitivity to near IR and visible lasers is primarily attributable to the marking component(s) of the metal oxide layer, namely CuO, MoO3, WO3, V2O5, Ag2O, or PbO2 and sensitivity to UV lasers is primarily attributable to the host component(s) of the metal oxide layer, namely SnO2 or TiO2. Moreover, TiO2 or SnO2, or a combination thereof, in the coating layer enhances the contrast of the CO2 laser marks compared with uncoated mica. This result may be due to the fact that the coating oxide layer increases the selectivity of the laser induced carbonization and, as a result, its yield. The oxide layer may also become partially reduced upon laser irradiation, forming dark-colored sub-oxides which increase the marking contrast.
The laser sensitive pigments of the invention are relatively thermally stable and exhibit good dispersability and minimal color effects. During calcination, the metal oxide coating layer crystallizes into nanometer sized crystals after calcination. Due to their small particle size, they show relatively high transparency and are therefore compatible with other colorants used in the final applications. Additionally, because the nanometer-sized particles are imbedded onto micrometer-sized particles, they can be processed more easily as micrometer-sized particles.
A preferred substrate particle is a platelet-shaped substrate. Although particle substrates with other geometries may be employed in accordance with the invention, platelet-shaped substrates are preferred because they tend to orient themselves when dispersed in a plastic matrix so that their larger surface faces are parallel to the object surfaces. This orientation maximizes the efficiency of the pigment particles to couple laser energy. The use of particle substrates is preferred over substrate free powders. A thin layer coating of metal oxides on transparent substrates reduces the necessary loading levels in comparison to substrate free powders, and thereby reduces the color effect of the oxides. A reduced color effect from the oxides makes the laser sensitive pigment additive more compatible with other colorants. Coated substrates are also more easily dispersed within the resin composition than substrate free powders.
Platelet-shaped substrates include, but are not limited to, the following materials: natural or synthetic mica such as muscovite, phlogopite, and biotite; other sheet silicates, such as talc, kaolin or sericite; glass platelets, silica flakes and alumina flakes. Mica particles are preferred because of its relatively high absorption of laser energy, and therefore a higher sensitivity when used in conjunction with CO2 lasers than other platelet-shaped substrates. Wet-ground muscovite is a preferred mica substrate. Mica particle sizes, as measured by light scattering methods, are preferably in the range of about 1 to about 150 xcexcm, more preferably in the range of about 5 to about 100 xcexcm, and most preferably in the range of about 10 to about 50 xcexcm. The particle size range may affect marking quality in that large particles tend to provide a higher contrast in the mark, but also decrease the definition and smoothness of the mark, particularly for fine marks.
The components of the coating layer are chosen to optimize laser sensitivity. Combinations of CuO with MoO3 or WO3 showed the best laser sensitivity marking contrast for the Nd:YAG laser, although any single component of these oxides also yielded fairly good marks. V2O5 may also be used in lieu of MoO3 or WO3, although the use of V2O5 may add a much stronger color to the material, which may be incompatible for use with light colored systems. Ag2O and PbO2 may be used as marking components in the metal oxide layer. However, their use may be constrained because of their higher toxicity. These marking oxides may be used alone as the coating layer on mica flakes. Preferably, however, the marking oxides are further mixed with TiO2 or SnO2 or a combination thereof, wherein the TiO2, SnO2, or the TiO2/SnO2 combination forms a host structure for the mixed oxide. The use of a host structure is advantageous in that the coating layer is more strongly bound to the substrate because TiO2 and SnO2 exhibit greater adherence to substrates than other oxides. The use of a host structure is also advantageous in that it increases the thermal stability of the mixed oxides and reduces the color effect of the mixed oxide because they are diluted by a colorless matrix. Additionally, the use of a TiO2 or SnO2 host structure enhances the sensitivity of the laser sensitive pigments to UV excimer lasers.
With an Nd:YAG laser, the marking sensitivity is mainly attributable the marking components of the coating layer, i.e. CuO, MoO3, WO3, or a combination thereof. However, the host components, i.e. TiO2 or SnO2, or a combination thereof, have some affect on the marking sensitivity. Materials with an SnO2 host layer show slightly lower sensitivity than materials with a TiO2 host layer. However, materials with an SnO2 host layer are more transparent than materials with a TiO2 host layer; therefore, materials with a SnO2 host layer may be more compatible with certain colorants in the plastic. The combination of CuO with MoO3 or WO3 shows greater sensitivity than either MoO3 or WO3 alone, although the combination does not show significant improvement compared to CuO alone. However, the combination is often preferable over CuO alone because the combination shifts the color of the product from a dark grayish color to a slightly yellowish color, which is more compatible with light colored systems. The Cu/W combination shows a lighter yellowish color than the Cu/Mo combination. Generally, the slightly colored additives add little color effect to the plastic objects because a very low percentage of the pigment additive is required to reach high sensitivity. The color and transparency of the additives can be fine-tuned by adjusting the oxide composition of the coating layer to suit the end application.
The weight ratio of host component to marking component in the mixed metal oxide layer is typically about 1 to about 0.1-0.4, preferably about 1 to about 0.2-0.4. For example, preferred mixed metal oxide layers are the following:
(a) 100 TiO2 and 10-40 CuO or 10-30 MoO3;
(b) 100 SnO2 and 10-40 CuO or 10-30 MoO3;
(c) 100 of a total of a mixture of TiO2 and SnO2 plus 10-40 CuO or 10-30 MoO3;
(d) 100 TiO2, 10-20 CuO, and 10-20 MoO3 or 10-20 WO3;
(e) 100 SnO2, 10-20 CuO, and 10-20 MoO3 or 10-20 WO3;
(f) 100 of a total of a mixture of TiO2 and SnO2, 10-20 CuO, and 10-20 MoO3 or 10-20 WO3;
(g) 100 TiO2, 10-20 CuO, 5-10 MoO3, and 5-10 WO3;
(h) 100 SnO2, 10-20 CuO, 5-10 MoO3, and 5-10 WO3;
(i) 100 of a total of a mixture of TiO2 or SnO2, 10-20 CuO, 5-10 MoO3, and 5-10 WO3.
Typically, mica particles of the invention have a metal oxide coating layer that is from about 30 nm to about 300 nm in thickness, preferably about 60 nm to about 150 nm.
The coating of the particle substrates with the oxide mixtures may be accomplished by precipitating the metal oxide components, either simultaneously or in sequence, onto the substrates in the medium of deionized water. The substrate material is first suspended in water, at a concentration of preferably about 50 to about 200 grams per liter. The solutions of coating precursor materials are delivered into the reactor at suitable inflow rates and pH values, under controlled temperature and agitation. It is preferable that precipitation of tungstate or molybdate marking components is accomplished in conjunction with precipitation of the host components. The relatively high solubility of tungstate or molybdate species in a broad pH range may result in incomplete precipitation if these oxides are precipitated individually in sequence. It is also preferable that the precursor solutions for WO3 or MoO3 are prepared by dissolving WO3 and/or MoO3 precursor materials such as in a NaOH solution, which is used as a neutralizing agent in the TiO2 and/or SnO2 precipitation. Typical WO3 and MoO3 precursor materials include anhydrous or hydrated oxides, acid or polyacid forms of tungstate or molybdate, alkoxides, and sodium or potassium salts of mono- or polytungstates or molybdates. The oxides, WO3 and MoO3, and the dihydrated sodium salts, NaWO4xe2x80xa22H2O and NaMoO4xe2x80xa22H2O, are preferred because of their broad availability and low cost. However, sodium or potassium salt may be used in a similar manner. The concentration of the NaOH solution can vary broadly but preferably is in the range of about 10% to about 30% in deionized water. The concentration of WO3 and/or MoO3 may be adjusted in accordance with the desired concentration of these components in the final coating layer. The preferred precursor materials for TiO2 and SnO2 precipitation are titanium tetrachloride and stannic chloride solutions, respectively, in a concentration range from about 300 to about 500 grams per liter. Other precursor materials that may be employed include Ti(SO4)2, TiBr4, and titanium alkoxides for Ti and Sn(SO4)2, SnBr4 and tin alkoxides for Sn. The preferred pH range for the precipitation is about 1.5 to about 2.5. At higher pH values, agglomeration of the substrates may occur, leading to poor coating quality.
The precipitation of copper species may be accomplished in various ways. A copper salt, preferably cupric chloride or alternatively cupric sulfate, nitrate, acetate or hydroxide, may be first dissolved in the Ti and/or Sn precursor solution in a suitable amount and then delivered into the coating reactor together with the Ti and/or Sn species for precipitation. However, during the first coating stage, where Ti and/or Sn oxides, as well as tungsten and/or molybdenum species, are essentially completely precipitated, the cupric salt is only partially precipitated due to its relatively high solubility at this pH range (1.5 to 2.5, as discussed above). To complete copper precipitation, the pH of the suspension needs to be slowly adjusted to the range of about 5 to about 6 with a NaOH solution. Alternatively, the addition of the copper precursor may be delayed until after the TiO2 and/or SnO2 precipitation, with or without W and/or Mo species precipitation, have been completed at lower pH. The cupric salt is then added to the suspension and afterwards the pH is slowly raised to the desired value. In the case that the CuO is to be incorporated into the TiO2 and/or SnO2 layer without the presence of W and Mo species, the Cu precipitation may also be completed at lower pH by using a reducing agent, for example, sodium tetrahydridoborate (NaBH4). In this process, a suitable amount of cupric salt is first dissolved in the Ti and/or Sn precursor solution and a suitable amount of NaBH4 is dissolved in the NaOH solution. Then the two solutions are introduced into the substrate suspension for precipitation in a similar way as described above.
Laser markable plastics and methods of laser marking of plastics are also contemplated as part of the invention. The laser sensitive pigments of the invention may be used in conjunction with a wide variety of polymers including, but not limited to, polyethylene, polypropylene, polystyrene, polyesters, polycarbonate, polyvinyl chloride, nylon, ABS, etc. For each type of resin, routine tests are needed to determine the optimal laser working conditions and pigment loading levels. Procedures and protocols for carrying out such routine tests are well known to those of skill in the art. Typical loading levels of laser sensitive pigment in the plastic composition are from about 0.1% to about 2%, preferably about 0.1% to about 1%, and more preferably about 0.25% to about 0.5%. Loading levels, as used herein, refers to concentrations of laser sensitive pigments in laser markable portions of the plastic material. Laser markable portions are portions of the plastic material that are accessible to the laser, For example, loading levels in the above described ranges may be localized near the surface of the object and the object may have minimal levels of laser sensitive pigment in areas that may not be contacted with the laser, such as the interior of the object.